postal district

a district of a British town or city that has been given a particular code(= group of letters and numbers to represent it) by the Post Office. Most postal districts have one or two letters for the town, and a number for the district. For example, the districts of Oxford are OX1, OX2, etc. In London, the letters represent an area. N1, for example, is in north London and SW8 is in south-west London.